Electrical strain sensors such as resistive foil sensors and piezoelectric-based sensors have been used. However, existing electrical strain sensors are generally susceptible to electromagnetic radiation and/or ionization radiation. They also have a limited transmission distance and they are not intrinsically safe. At least some of these drawbacks can be overcome by using optical strain sensors having Fiber Bragg Gratings (FBGs) embedded within an optical fiber to measure strain and/or temperature.
FBGs have been used extensively in the telecommunication industry. Indeed, FBGs can be used as wavelength selectable mirrors, where some wavelengths of light are reflected, while some other wavelengths of light are allowed to pass through. One FBG manufacturing technique involves shining a UV optical beam onto a core of an optical fiber to inscribe a periodic pattern within the core, over a small length thereof. The periodic pattern includes variations of the refractive index of the core of the optical fiber, which can act as reflective interfaces for at least some wavelengths, generally referred to as the Bragg wavelength λB. The Bragg wavelength λB of a FBG is a function of the periodic pattern which is inscribed in the core of the optical fiber. Accordingly, changing the spacing distance between two successive variations of the refractive index (i.e. the pitch) correspondingly changes the Bragg wavelength λB.
Although useful for managing different wavelengths in telecommunications, FBGs can also be used in strain sensing applications. Indeed, applying a strain to an optical fiber having a FBG inscribed in its core will modify the length of the optical fiber which will, in turn, change the pitch the Bragg wavelength λB of its FBG. This change can be monitored, enabling strain measurements to be performed optically.
By performing strain measurements using optical strain sensors, at least some benefits can be achieved over conventional electrical strain sensors. For instance, the measurement is no longer susceptible to electromagnetic interference, allowing these optical strain sensors to be positioned near highly electromagnetic interference emitting devices such as electric generators and/or transformers. Further, when the FBGs are manufactured in radiation hardened optical fibers, the possibility of monitoring temperature and/or strain in high ionizing radiation areas can be possible. Optical strain sensors also require no electrical energy at the point of measurement, and as a result, can be made intrinsically safe, enabling measurements to be performed in hazardous environments without introducing spark risks. Moreover, thermally stable FBGs can also be made, enabling strain measurements to be made at temperatures above 1000° C. for instance.
Although existing optical strain sensors have been found to be satisfactory to a given extent, there remains room for improvement; for instance, in providing optical strain sensors with increased resolution.